Because of the hostile environment to which it is subjected, the combustor liner of a gas turbine engine has presented one of the major maintenance problems of the engine. As is well known in this art, many schemes have been employed to cool the liner to increase its durability and lower its propensity to cracking occasioned by the severe thermal stresses.
The current state of the art is particularly exemplified by the apparatus disclosed in U.S. Pat. No. 3,978,662 granted to T. L. DuBell, T. C. Campbell and J. L. Thompson on Sept. 7, 1976 and U.S. Pat. No. 4,077,205 granted to F. C. Pane and D. Sepulveda on Mar. 7, 1978, the latter of which is assigned to United Technologies Corporation, the assignee of this patent application.
Both of these patents essentially show cooling schemes for louver constructed liners where U.S. Pat. No. 3,978,662, supra is a machined louver and U.S. Pat. No. 4,077,205, supra is sheet metal. In each instance the louver carries a lip that extends over the slot that develops or coalesces a film of cooling air which adheres to the wall of the liner for effectuating a cooling barrier for the combustor wall. Obviously, it is advantageous to design the cooling scheme such that the film propagates downstream the maximum distance. Hence, any increase in the expanse of the coalesced film results in a benefit to the durability of the liner.
For example, U.S. Pat. No. 4,077,205, supra installs posts or dimples adjacent the lip to prevent the lip from collapsing completely. The structure of U.S. Pat. No. 3,978,662 is designed to accommodate rearward mounted cooling air inlets for the cooling structure so that the cooling air is turned 180.degree. before being discharged into the combustor. This structure relies on static pressure as opposed to total pressure utilized by the present invention.
One theory that has been expounded for limiting the life of the liner is that the swirling effect of the cooling air persists in the coalesced film. This has the overall effect of impairing a uniform film generated by the louver lip. What seems to develop are localized hot spots adjacent to the swirling flow causing buckling and cracking.
Additionally, it has been found that some schemes expose certail wall surfaces of the liner at susceptible problem areas to both the hottest and coldest conditions. The high temperature difference obviously carries high thermal stresses working to the disadvantage of the liner.
I have found that by my invention, I can obviate or at least lessen the deleterious effects of the problems noted above. By virtue of the double loop cooling airflow, the swirling component is substantially reduced or eliminated effectuating a more uniformed coalesced cooling film that tests have shown to propagate further downstream than heretofore known cooling schemes, including the schemes shown in the aforementioned patents. Additionally, this scheme allows the air to pick up a given heat content that is in turn conducted to the stiffening ring that is exposed to the cold air side of the combustor. This heats the stiffening ring to a temperature intermediate to the cold air temperature and the hot combustor temperature resulting in a lower .DELTA.T and obviously, lessening the thermal stresses.